1. Field of the Invention
The present invention relates to a method of driving an optical deflection device that changes a direction of outgoing light relative to incident light, and preferably used in an image forming apparatus such as an electrophotographic printer or a copier, or a projection image-video display device such as a projector or a digital theater system.
2. Description of the Related Art
An optical switching device utilizing an electrostatic force and an optical deflection system using the optical switching device were published by K. E. Petersen in 1977 on “Applied Physics Letters, Vol. 31, No. 8, 1977, p521–523” (referred to as reference 1, hereinafter), in which a beam suspended at one end was electrostatically deflected to deflect reflected light so as to realize a switching function. Reference can also be made to Japanese Patent Gazette No. 2941952 (referred to as reference 2, hereinafter), and Japanese Patent Gazette No. 3016871 (referred to as reference 3, hereinafter).
Additionally, an optical switching element including a diffraction grating driven by an electrostatic force was published on “Optics Letters, Vol. 17, No. 9, 1992, p688–690” (referred to as reference 4, hereinafter).
Further, in Japanese Laid Open Patent Application No. 6-138403 (referred to as reference 5, hereinafter), an image forming apparatus using an optical deflection system is proposed in which digital micro-mirror devices (DMD) are arranged one-dimensionally or bi-dimensionally.
Additionally, as an example of the architecture of the above digital micro-mirror device, a digital micro-mirror device having a torsion beam (or cantilever-beam) architecture was published by L. J. Hornbeck in Proceedings of SPIE, Vol. 1150, p 86–102 (referred to as reference 6, hereinafter). The torsion beam (or cantilever-beam) digital micro-mirror device described by L. J. Hornbeck, similar to the present invention, has an inclined mirror portion, but, different from the optical deflection device of the present invention, the mirror portion has at least one fixed end.
Further, Japanese Laid Open Patent Application No. 2000-2842 (referred to as reference 7, hereinafter) discloses an optical deflection device that deflects a beam fixed at its two ends to a cylindrical shape and rapidly deflects the light.
The optical switching device utilizing a suspended beam, as disclosed in reference 1, or the cantilever-beam digital micro-mirror device disclosed in reference 6 suffer from drawbacks in that it is difficult to maintain stability of the beams, and the responding speed cannot be made high.
The torsion beam digital micro-mirror device suffers from a drawback in that the mechanical strength of the hinge (that is, the torsion beam) degrades after long-term use.
The optical switching devices disclosed in references 2 and 3 suffer from a drawback in that the wavelength of the incident light is limited.
The optical switching device disclosed in reference 7, which has a parallel gap between electrodes, and in which the beam fixed at two ends is deflected to a cylindrical shape, has the advantage in that it is capable of rapid deflection and thus high-speed response; however, because two ends of the beam are fixed, its driving voltage cannot be made low compared with the optical switching device utilizing a suspended beam, the cantilever-beam digital micro-mirror device, or the torsion beam digital micro-mirror device.
The above optical switching devices disclosed in the related art are of the suspended-beam type, torsion beam type, cantilever-beam type, or two-ends-fixed beam type. These optical switching devices of the related art have two or three electrodes. When the planar electrodes facing each other are set to be at different potentials, an electrostatic attracting force is generated, and this electrostatic force displaces mirror surfaces, thus performing operations of optical deflection.
Different from the above optical deflection devices, the present invention relates to an optical deflection device which includes plural electrodes (typically, four electrodes) formed on the same plane, and an electrically floating plate-shaped member (for example, a mirror) having a conductive layer and facing the electrodes. When a voltage is applied on adjacent electrodes, the potential of the electrically floating plate-shaped member becomes arbitrary, and an electrostatic attracting force is generated to act on the plate-shaped member. The electrostatic attracting force tilts and displaces the plate-shaped member, which has a mirror surface with a fulcrum member at its center, thus performing operations of optical deflection.
Usually, in the operation of driving such an optical deflection device, the mirror, as the plate-shaped member, is allowed to move, driven by a voltage in a specified region to deflect a reflection direction of incident light, then the voltage is removed to be set to its original value, and the mirror returns back to its original position due to the rigidity thereof.
Hence, in order that the mirror is not out of position, the movement of the mirror should be limited, and for this purpose, an end of the mirror is fixed by a rigid member. To utilize the rigidity, the rigidity is set to be large when thee driving speed is increased; accordingly, there arises a problem in that the driving voltage also increases. Additionally, because the rigid member has an elastic limit and a breaking limit, the allowed movement region of the mirror is quite limited.